1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
U.S. Pat. Nos. 5,339,737 and 5,379,698, the entire disclosures of which are hereby incorporated by reference, disclose a variety of lithographic plate configurations for use with imaging apparatus that operate by laser discharge (see, e.g., U.S. Pat. Nos. 5,385,092 and 5,819,661. These include "wet" plates that utilize fountain solution during printing, and "dry" plates to which ink is applied directly.
In particular, the '698 patent discloses laser-imageable plates that utilize thin-metal ablation layers which, when exposed to an imaging pulse, are vaporized and/or melted even at relatively low power levels. The remaining unimaged layers are solid and durable, typically of polymeric or thicker metal composition, enabling the plates to withstand the rigors of commercial printing and exhibit adequate useful lifespans.
In one general embodiment, the plate construction includes a first, topmost layer chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid. Underlying the first layer is a thin metal layer, which ablates in response to imaging (e.g., infrared, or "IR") radiation. A strong, durable substrate underlies the metal layer, and is characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Ablation of the absorbing second layer by an imaging pulse weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step. This, once again, creates an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer.
A considerable advantage to these types of plates is avoidance of environmental contamination, since the products of ablation are confined within a sandwich structure; laser pulses destroy neither the topmost layer nor the substrate, so debris from the ablated imaging layer is retained therebetween. This is in contrast to various prior-art approaches, where the surface layer is fully burned off by laser etching; see, e.g., U.S. Pat. Nos. 4,054,094 and 4,214,249. In addition to avoiding airborne byproducts, plates based on sandwiched ablation layers can also be imaged at low power, since the ablation layer does not serve as a printing surface and therefore need not be especially durable; a durable layer is generally thick and/or refractory, ablating only in response to significant energy input. The price of these advantages, however, is the above-noted post-imaging cleaning step.
In addition, the polymeric topmost coatings ordinarily required for the sandwiched-ablation-layer approach may exhibit less durability than traditional printing plates. For example, conventional, photoexposure-type wet plates may utilize a heavy aluminum surface capable of surviving hundreds of thousands of impressions. sandwiched-ablation-layer plates, by contrast, utilize polymeric topcoats that pass laser radiation through to the ablation layer. Hydrophilic polymers, such as polyvinyl alcohols, do not exhibit the durability of metals.
Difficulties can also be encountered when the sandwiched ablation layer is metal. First, a careful balance must be struck between reflection, absorption and transmission of imaging radiation. Metals exhibit an inherent tendency to reflect radiation; at the miniscule deposition thicknesses required for low-power imaging, however, a metal layer will absorb some radiation (which provides the ablation mechanism) and also pass some through. Increasing the thickness of such a layer augments laser power requirements not only through the addition of material, but also due to increased reflection of imaging radiation. The overall result is a maximum thickness limit, which restricts the ability to increase plate durability through thicker metal imaging layers.
Furthermore, thin imaging layers based on metal/non-metal combinations (e.g., metal oxides) can exhibit rigidity when deposited on a flexible polymeric substrate. Rigidity, too, increases with layer thickness, and excessively thick metal/non-metal layers will be vulnerable to fracture; for example, dimensional stress leading to fracture can occur as a result of heating and cooling, as when a thermoset coating is applied over such a layer and cured. A printing plate with an imaging layer damaged in this way will exhibit poor durability and possibly a loss of image quality.
Another type of problem that may arise in connection with sandwiched-ablation-layer plates concerns the ability to visually distinguish imaged from unimaged areas. Where the substrate is clear, the silvery metallic appearance of regions that have not received laser exposure may not contrast with the surface (e.g., a plate cylinder or inspection table) underlying the printing member, so that the imaged areas cannot be readily discerned. Similar difficulty may occur, for example, in certain constructions outlined in the '737 patent and U.S. Pat. No. 5,570,636 (the entire disclosure of which is hereby incorporated by reference) regardless of what underlies the construction. In particular, it is possible to laminate the above-described construction to a metal support that not only provides dimensional stability, but also acts to reflect transmitted imaging radiation back into the thin metal layer. Assuming clear substrate and laminating adhesive materials, however, the metal support, which remains intact after imaging, is likely to offer little contrast to the thin-metal layer.
Also as described in the '636 patent, it is possible to utilize thin-metal imaging layers over metal base supports without lamination. Although thermally conductive metal supports would dissipate imaging energy if disposed directly beneath the thin metal layer, the '994 application details constructions that concentrate heat in the thin metal layer, preventing (or at least retarding) its transmission and loss into the base support. To accomplish this, a thermally insulating layer is interposed between the imaging layer and the thermally conductive base support. Once again, assuming that the insulating layer is fabricated from a clear polymeric material, contrast between the thin metal layer and the metal base support will be minimal.
Printers have traditionally exploited contrast between imaged and unimaged plate regions to facilitate visual inspection. Typically, the press operator first utilizes the gross patterns to ensure that the plate corresponds to the current job, and that the series of plates on successive plate cylinders correspond to one another. He can then inspect the contrasting regions of the plates more closely, verifying proper overall imaging and the presence of key details prior to operating the press. The absence or a low level of contrast makes it difficult or impossible for a press operator to perform these identification and inspection activities by examination of the plate. Although the press operator can prepare a proof to obtain direct visualization of the plate image, this is time-consuming operation, particularly in a computer-to-plate environment.
Accordingly, a need exists for constructions that impart contrast between visually adjacent plate layers of similar tonality. One solution to this problem is set forth in U.S. Pat. No. 5,649,486 which is co-owned with the present application. The disclosed constructions contain a colorant that observably distinguishes the ink-accepting layer(s) from the ink-repelling layer(s), but it which does not substantially interfere with the action of the imaging pulses. In one embodiment, the printing member comprises a topmost layer, a thin metal imaging layer and a polymeric substrate comprising a material (such as a dispersed pigment, e.g., barium sulfate) that reflects imaging radiation and is tonally dissimilar to the thin metal layer. The colorant is chemically integrated, dispersed or dissolved within the polymer matrix of the substrate. Alternatively, because the topmost layer is removed as a consequence of the imaging process, it is possible to locate the colorant in this layer instead of (or in addition to) the substrate.
In a second embodiment, a construction comprising a topmost layer, a thin metal imaging layer and a polymeric substrate is laminated to a metal base support that is tonally similar to the imaging layer. A first version of this embodiment locates the colorant in the substrate layer, so that if the base support reflects unabsorbed imaging radiation, this will pass back to the thin metal layer through the colorant-containing substrate without significant absorption. In a second version, the colorant is located in the laminating adhesive. This second approach is advantageous in that it permits observation, for quality-control purposes, of the uniformity of the adhesive layer. Indeed, even in applications where visible contrast between imaged and unimaged plate regions is unnecessary (or perhaps even undesirable), a dye that is invisible under ambient light but observable under special conditions (e.g., which fluoresces under ultraviolet light) can be located within the adhesive layer. In a third version of this embodiment, the colorant is located in the topmost layer as discussed above. The colorant may be a dye, a pigment or a combination thereof.
Contrast can be useful for purposes other than visual proofing. For example, different colors can be used to distinguish different types of recording media, or for decoration, or for authentication. For these purposes, it may be desirable to utilize contrast media having color characteristics more complex than those of a simple dye or pigment.